Working with larger .stl wireframe files
You can reduce the mesh density in Blender by using a Decimate or Remesh modifier: create the modifier, change the settings until the mesh is as detailed as you need it, and then apply it. Don't forget to apply it or your mesh will still be very dense in edit mode.

If your mesh is too large for Blender to load at all, you could try using Meshlab, which is also free and open source and works nicely with .stl files. This video shows how to decimate a mesh using Meshlab.